What is Larmor frequency?

Example 8.4 (The common nuclei)

For a spin-$\frac12$ nucleus of moment $\mu$, $\gamma/2\pi = \mu/(Ih) = 2\mu/h$. With the measured moments: $\ce{^1H}$ $42.58\,\mathrm{MHz}/\mathrm{T}$, $\ce{^{19}F}$ $40.08\,\mathrm{MHz}/\mathrm{T}$, $\ce{^{31}P}$ $17.25\,\mathrm{MHz}/\mathrm{T}$, $\ce{^{13}C}$ $10.71\,\mathrm{MHz}/\mathrm{T}$, $\ce{^{15}N}$ $-4.32\,\mathrm{MHz}/\mathrm{T}$. A “$400\,\mathrm{MHz}$ spectrometer” has $B_0 = 9.4\,\mathrm{T}$: its protons resonate at $400.2\,\mathrm{MHz}$, its carbons at $100.7\,\mathrm{MHz}$.

Example 8.22 (An MRI image)

In magnetic resonance imaging a field gradient makes the Larmor frequency of the water protons depend on position, so that the Fourier transform of the signal is a map of where they are. Contrast comes from relaxation: the repetition time and echo delays are chosen to weight the image by $T_1$ or by $T_2$, which differ between tissues; contrast agents containing gadolinium shorten the $T_1$ of nearby water (Chapter 24).